A hot melt adhesive typically melts sharply upon heating and flows substantially freely for application to adherends which are being bonded. Upon cooling the adhesive hardens forming the desired bond.
Hot melt adhesives have been formulated for a variety of applications, including, for example, such applications as bonding papers, wood, textiles, and corrugated paperboard. Typical adhesive compositions often contain tackifying resins and oils or waxes to, for instance, modify the viscosity of the composition or improve its adhesion to substrates. In some instances, a second polymer or elastomer may be added to modify certain properties of the adhesive.
For use in bonding such articles as machine parts and electronic components, which may be made with a variety of materials including metals and plastics, an adhesive should provide a bond having sufficient thermal and mechanical shock resistance, and shear strength. In the case of electronic or electrical components, an adhesive, in order to be suitable for such uses as attaching electronic elements to a circuit board, should provide the desired electrical properties as defined by the application, e.g., low tendency to degrade the other materials of the components, high degree of resistivity, etc., as well as have a melting point such that the composition may be applied at temperatures low enough so as to not damage the other elements of the electronic component yet such that the bond will not be broken by the temperature required for other processes to which the component is subjected during fabrication and subsequent use.
Japan Kokai No. 60-120775 (Inaba et al.) discloses a hot melt adhesive comprising ethylene/propylene copolymer, atactic polypropylene, tackifier, and thermoplastic elastomer such as styrene-butadiene-styrene as a filler. Such compositions typically do not provide sufficient creep resistance, particularly under elevated temperatures. Furthermore, the electrical characteristics of such compositions may not be within ranges typically desired for use in electronic components.
Japan Kokai No. 58-23865 (Okude et al.) discloses hot melt adhesives toughened with particles of vulcanized rubber (5 to 200 mesh) to improve the elasticity of the adhesive and the bonds provided therewith. Adhesive compositions of the type disclosed in the reference typically do not achieve the degree of thermal and/or mechanical shock resistance, and shear strengths, particularly at low temperatures, that may be desired.